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XXXV. Photo-chemical Besearches. — Part IV. By Eobeet Bunsen, Professor of 
Phemistry at the University of Heidelberg, and Henry Enfield Eoscoe, B.A., Ph.P., 
Professor of Chemistry at Owens College, Manchester. 
Eeceived May 26, — Eead May 26, 1859. 
The measureless store of energy which Nature has amassed in the sun’s body flows in 
an unceasing current as solar rays throughout the universe. 
The laboui expended on the earth s surface in the maintenance of the animal and 
vegetable creation, and in the production of geological change, is derived, almost 
exclusively, from this source. 
Those of the sun’s rays which vibrate most slowly, and form the red portion of the 
solai spectrum, including the rays wsible and invisible which surround them, give rise 
by their absorption, more especially to the thermic actions observed on the surface of 
the earth, and in both the fluid zones which as ocean and atmosphere encircle the solid 
crust of our planet. These rays constitute the sources of heat which, in those grand 
processes of distillation and atmospheric deposit, have effected those vast transformations 
of the earth’s crust, by the study of which we obtain some idea of the immensity of the 
sun’s action exerted during geological ages upon our globe. 
Of a totally different kind, on a scale less magniflcent, but not less important, are the 
effects mainly produced by the more highly refrangible and more rapidly vibrating 
portions of the solar rays. These rays exert the most marked influence upon the chemical 
changes on which the vegetable world depends; and are therefore of the greatest 
importance as regards the character and geographical distribution of organic nature. 
Although the atmospheric phenomena regulating the amount and distribution of the 
chemical action of light on the earth s surface have not as yet been systematized to the 
same extent as the thermic, electrical, and magnetic phenomena of meteorology, the 
reason is not so much that their importance has been overlooked, as that the difficulties 
which surround an exact investigation of the subject have up to the present time proved 
insurmountable. If in the following research we have attempted to clear a path towards 
this new held of meteorological inquiry, we trust that these acknowledged difficulties 
may in some degree pro’s e an excuse for the many failings which our investigation 
presents. 
All experiments on the above subject can be of no value until some method has been 
discovered for expressing the chemical action of light in terms of some general and 
comparable measure. To this point, therefore, we were first obliged to direct our 
attention. 
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